1. Field of the Invention
The present invention relates generally to probe-based surface characterization and metrology instruments such as scanning probe microscopes (SPMs), and more particularly to an apparatus and method for improving metrology measurements of vertical dimensions by accounting for errors inherent in the scanning action of such instruments. The invention is particularly well suited for measuring pole tip recession on magnetic reading and/or writing heads.
2. Discussion of the Related Art
The present invention is relevant to measurements of the heights of features relative to a plane or surface whose area may be large with respect to the features or which may be a relatively large distance away from the features. These measurements, which will be referred to as "vertical metrology" measurements, must be performed with high accuracy for the successful development and manufacture of many modern devices such as data storage devices and semiconductor devices which must be manufactured to exacting tolerances to meet goals for data capacity with minimum expense. Precise vertical metrology measurements are required if the manufacturer is to be assured that these exacting tolerances are being maintained.
An important example of a vertical metrology measurement is the measurement of the recession of the pole tips of a recording head of a hard disk drive. The pole tips are the portions of the sensing or read/write element of the recording head which interface magnetically with the recording medium. During operation, it is desirable to minimize the spacing between the pole tips and the magnetic layer of the hard disk, thereby maximizing the signal-to-noise ratio obtained from the read element and the areal density of the data that can be written to the disk.
This spacing is in part determined by the "flying height" of the recording head, defined as the distance between the "slider", or body, of the recording head and the hard disk during operation. The nominal flying height H.sub.NOM of the recording head is determined by the "air bearing surface" (ABS) of the slider on which the read/write element is mounted. The modern trend in hard drive design is to reduce the flying height as much as possible, thereby bringing the pole tips closer to the medium and improving performance. Flying heights have decreased over the years from 100 nm or more to a current value of about 25 nm, and are expected to drop into the sub-nanometer range in future generations.
Another factor contributing to the spacing between the pole tips and the recording medium is the "pole tip recession" (PTR). On most recording head sliders, the pole tips are positioned laterally several microns behind the ABS, toward the trailing edge of the slider. Pole tip recession (PTR) is defined as the height difference between the ABS and the exposed surface of the pole tips. The pole tips usually are recessed with respect to the ABS, which positions them further from the hard disk during operation. This leads to an effective flying height H.sub.EFF which is greater than the nominal flying height H.sub.NOM. Manufacturers are 1) seeking to minimize pole tip recession so as to keep the pole tips as close as possible to the disk, thereby optimizing performance, while 2) seeking to assure that the pole tips are in fact slightly recessed so as to prevent damage to the pole tips in the event of slider contact with the disk. Pole tip recession historically has been smaller than the nominal flying height, but is becoming more significant as flying heights shrink.
Pole tip recession occurs because the ABS and the pole tips of the read/write element wear differently during polishing and other manufacturing processes due to differences in hardness between the slider and the pole tips. The ABS typically consists of a relatively hard ceramic material (such as Al.sub.2 O.sub.3 --TiC), whereas the pole tips are made of a much softer permalloy (such as Ni--Fe) surrounded by a sputtered alumina. The soft pole tips therefore wear more than the hard ABS during polishing and other manufacturing operations, resulting in pole tip recession.
While recession of the pole tips occurs naturally during these processes, maintaining the necessary tight tolerances for PTR and other characteristics requires stringent process control. This in turn relies on obtaining precise and accurate measurements of PTR. Today's data density goals require measurements of PTR to an accuracy and repeatability of about 1 nm for development and process control. It may also be desirable to measure other characteristics of the pole tips, an example being surface roughness.
Previously, pole tip recession has been measured with optical instruments such as optical profilers or interference microscopes (interferometers). One such instrument is a 3D surface profiler manufactured by Zygo Corporation under the name "MAXIM GP." This instrument combines phase measuring interferometry and optical microscopy to generate 3D surface profiles. Incoming light is split inside the interferometer of the instrument so that one beam goes to an internal reference surface and the other to the sample. The reflected beams recombine inside the interferometer, undergo constructive and destructive interference, and produce the light and dark fringe pattern common to interferometers.
Optical instruments such as interferometers provide repeatable measurements of the heights of pole tips and other features as discussed, for example, in 1) Nadimpalli, "Pole Tip Recession: Investigation of Factors Affecting its Measurement and its Variation with Contact Start-Stop and Constant Speed Drag Testing", Transactions of the ASME, Vol. 117, October 1995 and 2) Smallen "Pole Tip Recession Measurements on Thin Film Heads Using Optical Profilometry with Phase Correction and Atomic Force Microscopy", Transactions of the ASME; Vol. 115, July 1993. However, their images suffer offsets due the differences of materials within the sample. For example, the Ni--Fe alloy of the pole tips of a typical recording head will introduce a phase shift in the optical wavefront that is different from that produced by the surrounding sputtered alumina substrate material. This phase shift difference results in a spurious offset of up to several nanometers in the apparent height of the pole tips. Similar offsets are introduced by thin coatings of substances such as synthetic diamond which are often applied to the recording head to improve wear characteristics. These offsets can be corrected mathematically or empirically to an extent, but not with the precision (on the order of 1 nanometer) demanded by manufacturers of modern recording heads. Obtaining vertical metrology measurements with the accuracy required for recording heads and other modern devices therefore is difficult using optical instruments such as interferometers.
Another disadvantage of interference microscopes and other optical instruments is that their lateral resolution is limited by the diffraction of light (typically about 500 nanometers). This resolution is not sufficient for accurate measurement of the smallest features in the latest and future generations of recording devices. For example, pole tips on recording heads are expected to reach sub-micron lateral dimensions in the near future. In addition, magneto resistive sensors used on some recording heads are so narrow (on the order of 100 nanometers wide or less) that they typically cannot be detected by interference microscopes.
SPMs recently have been used for metrology measurements on data storage devices; semiconductors, and other devices. An SPM includes a probe that is scanned in a raster pattern over a surface and that measures an interaction between the probe and the surface. This interaction is monitored to produce an image of a characteristic of the sample such as its surface topography.
An important class of SPM is the atomic force microscope (AFM) which is a type of SPM in which the probe is mounted on a flexible cantilever. Interactions between the probe and the sample influence the motion of the cantilever, and one or more parameters of this influence are measured to generate data representative of the sample's surface topography. AFMs can be operated in different modes including contact mode, TappingMode, and non-contact mode. In contact mode, the cantilever is not oscillated, and cantilever deflection is monitored as the probe tip is dragged over the sample surface. In TappingMode (Tapping and TappingMode are trademarks of Digital Instruments, Inc.), the cantilever is oscillated mechanically at or near its resonant frequency so that the probe tip repeatedly taps the sample surface, thus dissipating energy and reducing the probe tip's oscillation amplitude. The oscillation amplitude indicates proximity to the surface and may be used as a signal for feedback. U.S. patents relating to Tapping and TappingMode include 5,226,801, 5,412,980 and 5,519,212, by Elings et al., all of which hereby are incorporated by reference. In the non-contact mode, attractive interactions between the probe tip and the sample (commonly thought to be due to Van der Waals's forces) shift the cantilever resonance frequency when the probe tip is brought within a few nanometers of the sample surface. These shifts can be detected as changes in cantilever oscillation resonant frequency, phase, or amplitude, and used as a feedback signal for AFM control.
Whether in contact mode, TappingMode, or non-contact mode, feedback is typically used during AFM scanning to adjust the vertical position of the probe relative to the sample so as to keep the probe tip-sample interaction constant. A measurement of surface topography may then be obtained by monitoring a signal such as the voltage used to control the vertical position of the scanner. Alternatively, independent sensors may monitor the position of the tip during scanning to obtain a map of surface topography. Measurements can also be made without feedback by monitoring variations in the cantilever deflection as the probe moves over the surface. In this case, recording the cantilever motion while scanning results in an image of the surface topography in which the height data is quantitative.
AFMs are capable of ultra-high resolution mapping of surface topography with vertical resolutions less than an Angstrom and with lateral resolutions on the nanometer scale. The vertical resolution meets or surpasses optical techniques, while the lateral resolution far surpasses them. AFMs are used successfully in numerous applications for measuring surface micro-roughness and local feature sizes. At first blush, AFMs may appear to be well suited for pole tip recession measurements and other vertical metrology measurements. However, AFMs have difficulties obtaining vertical metrology measurements with high accuracy because of problems inherent in their operation.
One problem encountered when attempting to take vertical metrology measurements using an AFM arises from imperfections in the scanning motion or in the detection of the probe's motion during scanning. For example, AFM scanners typically use a piezoelectric material formed into a tube geometry with patterned electrodes to provide lateral (XY) scanning motion as well as vertical (Z) motion. As the probe tip is moved to effect a scanning operation, the scanner approximates a "pendulum" motion so that the probe tip is lifted slightly from the sample surface as it moves away from the point of scan origin. The AFM attempts to compensate for this tip lift by controlling voltages to the actuator to extend the probe towards the sample so that the tip tracks the surface. As a result of this pendulum motion and resultant feedback compensation, images of flat surfaces appear curved or "bowed." A typical magnitude of this "bowing" effect is on the order of ten nanometers vertical excursion for a fifty-micron lateral scanning motion. A similar effect may also be caused by coupling of X and Y with some Z motion due to imperfections in the piezo or other actuators. This curvature precludes the precise determination of a useful reference plane or surface, and accurate metrology measurements of very small vertical dimensions such as pole tip recession are therefore very difficult.
Other problems may be encountered when attempting vertical metrology measurements using an AFM. For instance, the piezoelectric material commonly used as the actuator for the AFM scanner typically exhibits hysteresis in its motion. The hysteresis produces a difference in the scan data between probe tip movement in one direction (e.g., left to right) in the raster scan vs. the opposite direction (right to left). Imprecision in the detection of the cantilever motion may also lead to errors in scanning. Scanners can also "age", i.e., their characteristics, including bow and hysteretic effects, can change significantly with time. These factors can also preclude accurate vertical metrology measurements. Furthermore, the repeatability of the measurements may be compromised because hysteresis and aging cause them to change with time, sample tilt, and other factors, making pre-calibration difficult at the level of accuracy needed.
Conventional techniques used to remove instrumental error typically are not successful in removing the AFM scanner errors and detection idiosyncrasies like those described above. One such conventional technique involves modeling the instrumental error as a simple mathematical function. For example, one can compute the best fit of the scan data to a theoretical surface defined by a polynomial or other simple function. Subtracting this theoretical surface from the scanning data then removes part of the scanning error. In the case of an AFM, however, the bow, hysteresis, and detection errors often are not accurately described by simple mathematical functions. Furthermore, the fitting step can be compromised by features that deviate from a smooth surface, perhaps the very features requiring measurement. Fitting and subtraction therefore do not lead to improvements in accuracy sufficient for many applications (such as measuring pole tip recession) which require an accuracy on the order of 1 mn.
Another type of attempt to correct for instrumental error uses reference subtraction. In this technique, a reference scan is made of a standard sample. A likely reference sample candidate would be one having a flat surface such as that of a cleaved or polished silicon wafer. This scan then is subtracted from all data scans of subsequent samples. See U.S. Pat. No. 5,283,630 by Yoshizumi. This technique is commonly also used with interferometers to correct for optical imperfections. In the case of the AFM, however, hysteresis leads to scanner idiosyncrasies that depend on overall sample tilt which may vary significantly from sample to sample. These errors therefore will vary from a reference sample to a new sample and from scan-to-scan. Hence, a "standard sample" does not exist, and it is not possible to remove scanning errors using standard reference subtraction.
These issues can also apply to probe-based instruments other than SPMs, which are used for surface characterization, or other forms of metrology. For example, stylus profilometers have been used to measure characteristics of recording heads and other devices. Profilometers measure surface features by scanning a stylus over a sample surface. However, profilometers typically do not use feedback for scanning, and the profilometer stylus is not as sharp as an AFM probe. As a result, profilometers typically have a resolution inferior to SPMs, a greater noise floor, and a greater force of tip-sample interaction. Profilometers have nonetheless found many uses in the manufacture of devices such as recording heads.
Profilometer data can suffer many of the same idiosyncrasies as those of SPMs discussed above. In particular, profilometer data of a flat surface can appear curved, and vertical metrology measurements such as that of PTR can be compromised. As with SPM, conventional correction techniques can fail to correct adequately for these idiosyncrasies. For this reason, the Objects, Embodiments, and Claims of this patent should be understood to apply to profilometers, and other relevant probe-based surface characterization and metrology instruments, as well as to SPMs.
Assuming an instrument such as an SPM can be made to produce adequate data for PTR or other vertical metrology measurements, an additional need of manufacturers is the automation of these measurements. Automation increases measurement throughput and minimizes operator intervention, both important considerations for high-volume production.
A number of factors must be considered for automation of SPM measurements of pole tip recession or similar vertical metrology measurements. For example, the SPM must be engaged in a precise location relative to features of interest such as pole tips. Furthermore, once the SPM data is acquired, the actual measurement results must be determined. For example, the pole tip recession can be measured from the SPM data by 1) placing cursors or masks on the image so as to encompass the pole tips, and then 2) determining the average height or other mathematical characteristics of those areas from the image data. This process requires that the pole tips be identified and that their positions relative to the scan boundaries be determined precisely, thus allowing them to be measured without operator intervention.
SPMs have evolved to include automation functions that address these automation issues. A notable example is pattern recognition, an image analysis capability that allows features of interest to be automatically located within an optical image of a sample surface or within SPM data. Pattern recognition often uses a correlation analysis to "find" features by comparing them to a previously taught model. Success typically requires unambiguous feature shapes as well as clear contrast.
Pattern recognition can be used to automate the acquisition of SPM data. Many SPMs include an optical microscope integrated with the SPM, which can thereby produce an image of the sample, and this image can be digitized. Because the material comprising the pole tips of a recording head is highly reflective, the pole tips are distinct in such an optical image. Pattern recognition can be used in conjunction with the optical image to help position the sample relative to the tip so that subsequent SPM data scan encompass features of interest. For example, prior to engaging the SPM, pattern recognition can be used in conjunction with the optical image to determine the position of the pole tips. This position can vary from sample to sample due to variations in recording head dimensions or errors in positioning the sample on the SPM stage. In response to the pattern recognition output, the stage can adjust the position of the recording head so that the SPM engages in a precise location relative to the pole tips. The scan data will then encompass the pole tips and other regions of the recording head as needed.
Another way pattern recognition can be used to automate vertical metrology is in the measurements taken on the scan data once it is acquired. In the case of PTR, pattern recognition could, in principle, locate features of interest such as pole tips within the AFM image data. Due to slight errors in sample positioning before scanning, the positions of the pole tips within the data image can shift slightly from scan to scan. Small errors of this kind can occur even when fine positioning of the sample is performed prior to scanning as discussed above. Pattern recognition could account for these shifts, and allow cursors or masks to be placed on the pole tip regions and so allow the measurement to be taken automatically. Knowledge of location of features within a data scan can also be used is to adjust the sample position so that subsequent SPM data scans are positioned accurately on the sample.
In practice, however, using pattern recognition to locate the position of the pole tips within AFM scan data is difficult. This is because the pole tip recession may be very small, resulting in weak image contrast which prevents the pole tips from appearing distinctly within the AFM image. Surface roughness, polishing scratches, and lapping debris on the recording head can further obscure the pole tips. As a result, pattern recognition has difficulty identifying and locating the pole tips within the AFM scan, and fully automated PTR measurements are difficult using AFM data alone.